Gallium nitride (GaN) high electron mobility transistor (HEMT) devices have high breakdown voltages and high cutoff frequencies. Such devices are accordingly useful in high power and high efficiency amplifiers and other devices for high frequency communications and other high frequency applications. HEMT devices are also known as heterojunction field effect transistor (HFET) devices in reference to the derivation of a transistor from a heterostructure.
AlGaN/GaN heterostructures are capable of high speed switching and present high breakdown voltages because of the high breakdown field strength of GaN and the high electron sheet density of the AlGaN/GaN heterojunction. The high breakdown field strength and high electron sheet density arise from the wide 3.4 eV bandgap of GaN. This bandgap is much wider than the bandgap of other semiconductor materials, such as Si (1 eV bandgap) and GaAs (1.6 eV bandgap). Such GaN heterostructures are accordingly often used in devices calling for highly efficient operation.
GaN HEMT devices have suffered from leakage current and trap-related phenomena, such as current collapse and quiescent current drift. GaN HEMT devices may degrade upon the formation of defects in an AlGaN barrier layer of the device. The defects provide a leakage path for electrons and effectively lower the Schottky barrier height of the gate. Current collapse is a frequency dispersion phenomenon, and may result from surface and buffer traps.
Field plates have been used to reduce the electric field at the gate edge. The reduction in the electric field in that region may address issues of device degradation and current collapse. GaN caps have also been used to reduce the electric field at the gate edge and improve surface morphology. Silicon nitride films have also been used to decrease the influence of surface traps in the interest of addressing current slump. Despite these efforts, gate leakage remains a problem for AlGaN/GaN HFET devices with Schottky gates.